Revert 7151 "Revert 7114 "Expose VideoEncoders with webrtc/video_encoder.h.""
Revert 7151 "Revert 7114 "Expose VideoEncoders with webrtc/video_encoder.h.""

Re-lands r7114 after landing r7204 to adress the compile error causing
the rollback in r7151.

BUG=3070
TBR=henrikg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/28489004

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: ab990ae43a2b84b103cb3c50bc38502375c13e68

sippet/talk,sippet/talk,sippet/talk,sippet/talk,sippet/talk